The present invention relates to methods for reducing stress on the tips of blades in a gas turbine engine, and more specifically, to a method for reducing stress on blade tips intended to contact a circumferential seal.
Gas turbine engines include a series of compressor and turbine blades that rotate about a central axis of the engine. The efficiency of the compressor and of the engine depends in part on the volume of compressed air that leaks through the interface between the compressor blades and the surrounding circumferential shrouds or seals. Similarly, the efficiency of the turbine section is affected by leakage of the expanding products of combustion past the circumference of the turbine blades. Engine efficiency can be increased by decreasing the size of the gap between the tips of the compressor or turbine blades and the cooperating circumferential seal to reduce leakage past the blade, seal interface.
One prior art method used to reduce loss between the blade tips and the cooperating circumferential seal employs abradable seals. In this structural configuration, the circumferential seal that surrounds the blades is formed of a material that can readily be worn away or abraded by contact with the blade tips. In order to seat the blades in the seal, the blades are rotated so that the tips of the blades rub against or abrade the outer seal until a proper fit is achieved. This method of seating the seal produces a close tolerance fit that reduces air losses through the seal. The use of abradable outer seals has been successful in increasing engine efficiency.
In the past, the abradable outer seals were commonly formed of a material commonly referred to as "fiber" metal. Fiber metal is a very soft, easily abradable material that allowed the blade tips to cut into the seals without causing significant damage or wear to the blade tips.
In modern turbine engines, even closer tolerances between the blades and seals than have been achieved in the past are desirable to further increase engine efficiency. To achieve this, outer seals are being formed from harder, denser and more durable materials capable of producing closer tolerances and greater seal life. However, the use of such materials contributes to increased damage and wear of the blade tips during the seating process. Physical contact between the blade tips and the harder seal materials tends to abrade and damage the blade tips. This damage in turn contributes to increased blade wear and increased metal temperatures which can lead to failures due to crack initiation and propagation. Tip abrasion reduces overall blade life and affects the aerodynamic configuration of the blade, thus decreasing engine efficiency.
One method known to reduce blade tip wear during seal seating in the harder seal material is to apply an abrasive coating to the blade tips as shown in FIGS. 1-2. An abrasive coating 10 is applied to the tip 12 of a blade 14. The abrasive coating is a hard material that helps the blade to cut into the abradable seal without causing significant wear or damage to the abrasive coating 10 or blade tip 12. Often, the abrasive coating includes abrasive particles 16 that are trapped within some type of metal matrix. The abrasive particles may protrude from the tip coating in order to assist the blade tip in cutting into and seating in the abradable seal.
Two examples of methods to apply an abrasive tip coating are disclosed within U.S. Pat. Nos. 5,074,970 and 4,169,020, the specifications of which are incorporated herein by reference. Many different materials can be used as abrasive tip coatings, including nickel or aluminum oxide, cubic boron nitride, various abrasive carbides, oxides, silicides, nitrides, and other materials suspended in a matrix. Such coatings can be applied by electroplating, plasma spraying, or in accordance with other methods commonly known and practiced by those of ordinary skill in the art.
While it is true that abrasive tip coatings reduce blade tip wear and damage during the seating of the abradable seal, contact between the tip of the blade and the abradable seal add to the magnitude of the already high stress levels present at the tip of the blade during engine operation. As illustrated in FIG. 3, during engine operation, the tip of the blade tends to deform at a resonance frequency. A representative mode shape for a typical tip resonance bending mode is shown in phantom in FIG. 3. In the mode shown, the leading 17 and trailing 18 edges of the blade deform alternately inwardly and outwardly during resonance introducing bending stress through the thickness of the blade. As illustrated in the cross section of the blade shown in FIG. 4, the absolute magnitude of the stress 19 increases as one moves from the center of the blade toward either of the opposing surfaces. Contact between the tip of the blade and the circumferential seal further increases the magnitude of the stress at the blade tip and contributes to blade failure due to crack initiation and propagation.
Tip coatings further increase the magnitude of the stress at the blade tip because each of the abrasive particles 16 (FIG. 2) can act as an individual stress riser on the blade tip. These stress risers in turn increase the chance of blade failure due to crack initiation and propagation.
The use of abrasive tip coatings is especially detrimental to the fatigue life of blades formed from highly crack sensitive materials, such as titanium. Titanium is one of the preferred materials from which compressor blades are manufactured, due to its high strength, temperature tolerance, stiffness, and low density. Therefore, fatigue strength reductions caused by tip coatings are particularly important in the production of more efficient, long life turbine engines made with such materials.
As the understanding of the aerodynamic processes occurring within gas turbine engines improves, it will become even more important to reduce the detrimental effect of tip coatings on overall blade life. Blades are becoming increasingly thinner and more sharply contoured in order to increase aerodynamic efficiency. Thus, new blade configurations have less surface area on the blade tips on which to apply abrasive coatings. This decrease in surface area may require development of new abrasive coatings for seating the blades in the abradable seals.